The present invention relates to a method for fabricating compound semiconductor devices. More particularly, it relates to a method including an anneal of a composite formed of a compound semiconductor substrate, a SiO.sub.2 insulator film, and a second film containing a refractory metal.
Ion implantation is widely used as the process for doping, or adding small amounts of impurities to, compound semiconductors. That process is used because it has superior reproducibility and controllability, allowing the conductivity of the semiconductor to be increased greatly and to a precisely controlled extent.
To activate an ion-implanted region, a heat treatment at high temperature (annealing) is necessary. Because the constituent atoms of a compound semiconductor evaporate at high temperatures, however, special considerations are required for an annealing method applied to such semiconductors. For purposes of discussion, consider the case of the GaAs compound semiconductor.
High-temperature anneals applicable for a GaAs, ion-implanted region may be classified generally into two known processes. One process is the "capless" anneal, in which the anneal is done without an insulating film but under a pressurized gas atmosphere including AsH.sub.3 (arsine). The second process is the "capped" anneal, which uses an insulator such as SiO.sub.2, SiN, or the like as an encapsulant film.
In the capless anneal, as contrasted with the capped anneal, the activation efficiency is excellent and a steep carrier concentration profile may be obtained. Arsine is a toxic gas, however, which creates a difficult safety problem. Consequently, very expensive equipment is required to guarantee safety.
In the case of the capped anneal, on the other hand, the atmospheric gas is usually Ar (argon), N.sub.2 (nitrogen), H.sub.2 (hydrogen), or the like. Such gases are less dangerous than AsH.sub.3 and relatively inexpensive equipment may be used to perform the anneal. When an SiO.sub.2 insulator film is used, although adhesion with the GaAs substrate is satisfactory at high temperature, diffusion of Ga and As away from the GaAs substrate cannot be suppressed. Moreover, the capped anneal yields a poor activation efficiency, the carrier profile of the ion implanted layer shows a deep tail, and it is difficult to obtain the steep profile desired. When an SiN insulator film is used, as opposed to the SiO.sub.2 film, the diffusion of Ga and As away from the GaAs substrate can be arrested. The adhesion with GaAs is poor at high temperature, however, and it is difficult to reproduce and obtain a SiN film of high quality.